1. Field of the Invention
The present invention relates to an optical recording and/or reproducing apparatus, and, more particularly, to an optical pickup actuator designed to improve adaptability of an optical disc having a deflection error, a method of manufacturing the same, and an optical pickup and an optical recording and/or reproducing apparatus employing the same.
2. Description of the Related Art
In general, an optical recording and/or reproducing apparatus to record and/or reproduce information onto and/or from an information storage surface of an information storage medium, such as a recording surface of an optical disc, employs an optical pickup to perform recording and/or reproducing of information on the recording surface of the optical disc by radiating light onto and/or receiving light reflected from the recording surface while moving in a radial direction of the optical disc.
The optical pickup includes an actuator. FIG. 1 is a plan view of a conventional optical pickup actuator, and FIG. 2 is a side view of the optical pickup actuator of FIG. 1. Referring to FIGS. 1 and 2, the conventional optical pickup actuator includes a base 7, a holder 8 fixed onto the base 7, a blade 2 that is movably supported by a suspension 6 having one end fixed to the holder 8 and on which an objective lens 1 is mounted, a focus coil 3 and tracking coils 4 that are respectively mounted on the blade 2 so as to form paths through which currents flow to drive the objective lens 1 in a focus direction A and a tracking direction B, and a magnet 10 which generates a magnetic field that interacts with currents flowing through the focus coil 3 and tracking coils 4 to produce an electromagnetic force to drive the blade 2 and the yokes 9. In FIGS. 1 and 2, reference numeral 11 denotes a turntable on which an optical disc D is placed, and reference numeral 12 denotes a motor 12 rotating the turntable 11.
In the conventional optical pickup actuator having the above-described configuration, when current is supplied to the focus coil 3, the blade 2 is driven in the focus direction A by an electromagnetic force that is generated due to an interaction between the current and magnetic field generated by the magnets 10 and the yokes 9. In this case, since the focusing direction is determined according to the direction of the current flowing through the focus coil 3, a focal distance between the objective lens 1 and the recording surface of the optical disc D may be adjusted by controlling the amount of current flowing through the focus coil 3. Furthermore, the blade 2 is driven in the tracking direction B by an electromagnetic force. By controlling the amount of current flowing through the tracking coils 4, the objective lens 1 is allowed to precisely follow the desired track on the optical disc D.
Although a completely flat recording surface is ideal for recording, the optical disc is actually curved slightly upward or slightly downward so that the optical disc has a deflection error. The deflection error of the optical disc D acts as a focusing error with respect to the optical pickup actuator. That is, when the optical disc D with the deflection error rotates on the turntable 11, the focal distance between the objective lens 1 and the optical disc D varies according to an amount of the curvature of the optical disc. Thus, to compensate for variation due to the deflection error of the optical disc D, the optical pickup actuator performs control in the focus direction A.
Since the blade 2 moves in an arc by hanging on one end of the suspension 6, the blade 2 does not move precisely in either a vertical or a horizontal direction during focusing or tracking operations due to an error introduced within the manufacturing process or a problem with the structure of the optical pickup actuator. Instead, the blade 2 suffers a rolling phenomenon as shown in FIG. 2 in which the blade 2 sways from side to side or in backward and forward directions. Rolling, in which the blade 2 tilts about a rotary axis that is normal to a radial direction of the optical disc D as shown in FIG. 2, is called radial rolling. When the blade 2, gets close to the outer perimeter of the optical disc D, the blade 2 moves upward. In other words, when the objective lens 1 moves toward the optical disc D, radial rolling is in the positive (+) direction. Conversely, when the blade 2 moves downward, radial rolling is in the negative (−) direction.
FIGS. 3A-3C show three types of radial rolling. FIG. 3A shows a first type of radial rolling (hereinafter referred to as ‘type A radial rolling’). Referring to FIG. 3A, when the blade 2 moves upward, i.e., when the objective lens 1 moves toward the optical disc D, radial rolling is in the positive direction, and conversely, when the objective lens 1 moves away from the optical disc D, radial rolling is in the negative direction. FIG. 3B shows a second type of radial rolling (hereinafter referred to as ‘type B radial rolling). When the objective lens 1 moves toward the optical disc D, radial rolling is in the negative direction, and conversely, when the objective lens 1 moves away from the optical disc D, radial rolling is in the positive direction. FIG. 3C shows a third type of radial rolling (hereinafter referred to as ‘type C radial rolling’). Referring to FIG. 3C, radial rolling is in the same direction, either positive or negative, regardless of whether the objective lens 1 moves toward or away from the optical disc D.
Type A radial rolling is effective in reducing a deflection error since this type of rolling causes an optical axis C1 of the objective lens 1 to be almost perpendicular to the recording surface of the optical disc D when the optical pickup actuator performs focus control to adjust the position of the objective lens 1 with respect to the optical disc D having a deflection error. Conversely, the type B radial rolling results in an increase in the deflection error since the angle between the optical axis C1 of the objective lens 1 and the recording surface of the optical disc D exceeds 90 degrees. The type C radial rolling is effective for situations when either the objective lens 1 moves upward or when the objective lens 1 moves downward.
Among the above three types of radial rolling, the type A radial rolling is ideal for improving adaptability to an optical disc with a deflection error, and the type C radial rolling is not ideal but may be usable. However, type B radial rolling is not desirable because this type of rolling has a fatal effect on the recording/reproduction performance. However, in actuality, a nearly equal percentage of optical pickup actuators that exhibit the three types of radial rolling are fabricated as a result of assembling tolerances and various other factors. To solve the problem, only optical pickup actuators exhibiting the types A and C radial rolling are used while those having the type B radial rolling have been discarded as being defective. However, treating the optical pickup actuators exhibiting the type B radial rolling as defective results in low productivity.
When an optical pickup actuator is designed to have radial rolling characteristics that may cancel the deflection error of an optical disc during focusing operation, improving the control performance of the optical pickup actuator is possible. Thus, there is a need for a method of inducing desired radial rolling characteristics. That is, having a method of inducing type A radial rolling characteristics to improve the adaptability of an optical pickup actuator is relatively highly desirable.
An optical pickup actuator exhibiting the type A radial rolling may offset the deflection error of an optical disc when performing focus control by positioning the objective lens with respect to the optical disc since this type of rolling causes the angle between the optical axis of an objective lens and the recording surface of the optical disc to tend to be closer to 90 degrees.
To induce the type A rolling characteristics, the stiffness of each of inner and outer suspensions with respect to the objective lens may be varied. This approach has been proposed in U.S. Pat. No. 6,570,828.
The above-cited reference presents an example in which a movable part is tilted at a predetermined angle when being driven in the focus direction by changing the diameter of a suspension so that the stiffness of the suspension close to the inner perimeter of the optical disc is higher than that of the suspension close to the outer perimeter thereof or that an elastic force of the suspension close to the inner perimeter is suppressed.
As the speed of an optical recording and/or reproducing apparatus increases, the rotating frequency of an optical disc increases. Since the operating frequency of an optical pickup actuator in focus or tracking direction synchronizes with the rotating frequency of the optical disc, the actual operating frequency moves to a high frequency band.
The conventional method of inducing type A rolling characteristics is effective for a low frequency band extending below the first resonant frequency of the optical pickup actuator, but not for a high frequency band above the first resonant frequency. That is, offering the type A rolling characteristics by adjusting a stiffness of the suspension located at the inner and outer perimeters of the optical disc in the high frequency band is difficult. Thus, the conventional method cannot provide high adaptability to the deflection error of the disc for a high-speed optical recording and/or reproducing apparatus.